In this application, we will compare the responses of B cells from MRL/lpr lupus prone mice and non-autoimmune mice to receptor editing signals, testing the hypothesis that lupus-prone strains of mice have misregulated receptor editing responses. Using a short term in vitro culture system in which receptor editing can be induced by anti-idiotype antibodies, we have shown that MRL/lpr mice show much less upregulation of RAG mRNA following these receptor editing signals. We will use this culture system to determine if MRL/lpr mice show lower levels of receptor editing to low and moderate affinity soluble ligands. We will also determine if the immature B cells from MRL/lpr mice are less responsive to all BCR signals, or if the RAG upregulation response following BCR engagement is uniquely dampened. Editing at the H chain locus is controversial. We will determine if VH replacement and H chain editing is observed in MRL/lpr mice and/or normal mice, and if so, we will use the receptor editing cultures to determine if VH replacement is stimulated by receptor editing signals. Using these cultures, we will assess the accessibility at the V, D and J regions of both the H and L chain loci by chromatin immunoprecipitation with antibodies reactive with acetylated or methylated histones. We will compare the chromatin modifications at all of these loci in B cells from MRL/lpr and control mice before and after induction of receptor editing and in control cultures. Anti-DNA and other autoantibodies are often characterized by long H chain CDR3, sometimes containing two D regions. We have shown that V-D-D-J rearrangements with long CDR3 are made in adult MRL/lpr mice, but not in neonates, and we will test the hypothesis that TdT may be required for the production of these unusual V-D-D-J rearrangements. Alternatively, V(D)J assembly may be differentially regulated in newborns and adults. The information gained in these aims should give insights into the response of the various Ig loci to receptor editing signals, and how these processes may potentially be misregulated in B cells of autoimmune mice. [unreadable] [unreadable]